Optical sheet having a composite structure thereon and method to make the same

ABSTRACT

The present invention discloses a method of forming an optical sheet. The optical sheet comprises a substrate and a film. The substrate has a first surface and a second surface opposite to the first surface. The film has a third surface and a fourth surface opposite to the third surface. The third surface of the film is on the first surface of the substrate. The fourth surface of the film comprises a structure corresponding to a combination of a plurality of first convex shapes and a plurality of second convex or concave shapes superimposed on the plurality of first convex shapes.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/213,420 filed on Jul. 19, 2016, which claims priority ofU.S. Provisional Application Ser. No. 62/336,761 filed on May 16, 2016.This application also claims priority of U.S. Provisional ApplicationSer. No. 62/361,519 filed on Jul. 13, 2016. All of these applicationsare incorporated by referenced herein in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an optical sheet, and more particularlyto an optical sheet having a composite structure thereon.

2. Description of Related Art

The aim in the development of the LCD (liquid crystal display) is topursue lightness, thinness and high resolution. That is, the brightnessof the backlight module needs to be improved and the thickness of thebacklight module needs to be minimized. When the elements of thebacklight module are changed from “the top diffusing sheet/the top prismsheet/the bottom prism sheet/the bottom diffusing sheet” to “the topprism sheet/the bottom prism sheet/the bottom diffusing sheet”, it iseasy to see Newton's ring. Newton's ring results from light interferencebetween two adjacent interfaces and often occurs between the bottompolarizing sheet and the top prism sheet or between the top prism sheetand the bottom prism sheet. Newton's rings between the top prism sheetand the bottom prism sheet can be classified into two types:transmissive Newton's ring resulting from the incident backlight andreflective Newton's ring resulting from the incident environment light.The surface configuration of the backside structure of the top prismsheet can be optimized to destroy the interference of the incidentbacklight and the incident environment light so as to eliminate Newton'sring.

The backside structure of the top prism sheet is generally formed byembossing and photo cure. There are two methods for manufacturing thebackside structure of the prism sheet: the FTS (Fast Tool Server)process and the sandblasting process.

FIG. 1A illustrates a schematic cross-sectional view of the backsidestructure 11 of the prism sheet 10 formed by the FTS process. By the FTSprocess, each convex lump 12 of the backside structure 11 of the prismsheet 10 is smoothly-curved and thus the loss of the brightness isslighter; however, because the density of the convex lumps 12 issmaller, Newton's ring (especially reflective Newton's ring) is notcompletely eliminated. The haze of the convex lumps 12 formed by FTSprocess is about 10%. Because the convex lumps 12 formed by FTS processare more regular, Newton's ring easily occurs. In order to eliminateNewton's ring, the haze of the surface must be increased to be not smallthan 30%; however, the haze increases a lot and the brightness drops.

FIG. 1B illustrates a schematic cross-sectional view of the backsidestructure 21 of the prism sheet 20 formed by the sandblasting process.By the sandblasting process, the density of the convex lumps 22 islarger to have better ability of eliminating Newton's ring; however,because the surface of the backside structure 21 of the prism sheet 20is coarser, the haze increases and the brightness drops a lot.

Accordingly, the present invention proposes an optical sheet having acomposite structure thereon and its manufacturing method to overcome theabove-mentioned disadvantages.

SUMMARY OF THE INVENTION

The present invention provides an optical sheet and a method formanufacturing the optical sheet which has better anti-Newton's ringperformance (especially eliminate reflective Newton's ring) and betterbrightness performance.

In one embodiment, the present invention discloses an optical sheet,comprising: a substrate having a first surface and a second surfaceopposite to the first surface; and a film having a third surface and afourth surface opposite to the third surface, wherein the third surfaceof the film is on the first surface of the substrate and the fourthsurface of the film comprises a structure corresponding to a combinationof a plurality of first convex shapes and a plurality of second convexor concave shapes superimposed on the plurality of first convex shapes.

In one embodiment, the present invention discloses an optical sheet,comprising: a substrate having a first surface and a second surfaceopposite to the first surface; and a film having a third surface and afourth surface opposite to the third surface, wherein the third surfaceof the film is on the first surface of the substrate and the fourthsurface of the film comprises a structure corresponding to a combinationof a plurality of first convex shapes and a plurality of second convexshapes superimposed on the plurality of first convex shapes.

In one embodiment, the present invention discloses an optical sheet,comprising: a substrate having a first surface and a second surfaceopposite to the first surface, wherein the first surface of thesubstrate is a light input surface and the second surface of thesubstrate is a light output surface; a light enhancement layer disposedon the second surface of the substrate; and a film having a thirdsurface and a fourth surface opposite to the third surface, wherein thethird surface of the film is on the first surface of the substrate andthe fourth surface of the film comprises a structure corresponding to acombination of a plurality of first convex shapes and a plurality ofsecond convex or concave shapes superimposed on the plurality of firstconvex shapes, wherein the average size of the plurality of first convexshapes is larger than the average size of the plurality of second convexor concave shapes, wherein the average roughness (Ra) of the structureis 0.4˜0.45 μm.

The detailed technology and above preferred embodiments implemented forthe present invention are described in the following paragraphsaccompanying the appended drawings for people skilled in this field towell appreciate the features of the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the accompanying advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed descriptionwhen taken in conjunction with the accompanying drawings, wherein:

FIG. 1A illustrates a schematic cross-sectional view of the backsidestructure of the prism sheet formed by the FTS process.

FIG. 1B illustrates a schematic cross-sectional view of the backsidestructure of the prism sheet formed by the sandblasting process.

FIG. 2A to FIG. 2C respectively illustrate a schematic cross-sectionalview of a first optical sheet, a schematic cross-sectional view of asecond optical sheet and a schematic cross-sectional view of a thirdoptical sheet in the present invention.

FIG. 3A to FIG. 3D illustrate a preferable manufacturing method forforming the optical sheet in the present invention.

FIG. 4 illustrate that the first concave shapes are configured formaking corresponding convex regions having a first haze H₁.

FIG. 5A to FIG. 5B illustrate a real composite structure correspondingto the combination of a plurality of first convex shapes and a pluralityof second shapes.

FIG. 6 illustrates the composite structure can eliminate Newton's ring(especially reflective Newton's ring).

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The detailed explanation of the present invention is described asfollowing. The described preferred embodiments are presented forpurposes of illustrations and description and they are not intended tolimit the scope of the present invention.

In the following context, the haze is measured based on the combinationof the substrate and the composite structure (serving as light inputsurface) disposed on the substrate.

FIG. 2A to FIG. 2C respectively illustrate a schematic cross-sectionalview of a first optical sheet 100A, a schematic cross-sectional view ofa second optical sheet 100B and a schematic cross-sectional view of athird optical sheet 100C in the present invention. The optical sheet100A, 100B, 100C comprises a substrate 103 and a film 101. The substrate103 (e.g., PET substrate) has a first surface 103A and a second surface103B opposite to the first surface 103A. The substrate 103 can be aunitary (or homogeneous) substrate. The film 101 has a third surface101A and a fourth surface 101B opposite to the third surface 101A,wherein the third surface 101A of the film 101 is on the first surface103A of the substrate 103 and the fourth surface 101B of the film 101comprises a composite structure 102 corresponding to a combination of aplurality of first convex shapes 102X and a plurality of second shapes102Y superimposed on the plurality of first convex shapes 102X.Preferably, the film 101 can be a unitary (or homogeneous) film. Theaverage size of the first convex shapes 102X is larger than the averagesize of the second shapes 102Y. The density of the second shapes 102Y islarger than the density of the first convex shapes 102X. In oneembodiment, the fourth surface 101B of the film 101 comprises acomposite structure 102 corresponding to a combination of a plurality offirst convex shapes 102X and a plurality of second convex or concaveshapes 102Y (second convex shapes 102Y are preferred) superimposed onthe plurality of first convex shapes 102X. In the preferred embodiment,the fourth surface 101B of the film 101 comprises a composite structure102 corresponding to a combination of a plurality of first lenticularshapes 102X and a plurality of second convex or concave shapes 102Y(second convex shapes 102Y are preferred) superimposed on the pluralityof first lenticular shapes 102X. Preferably, the projection on the firstsurface 103A of the substrate 103 of each first lenticular shape 102X iselliptical. For increasing diffusion area, the adjacent first convex orlenticular shapes 102X overlap so as to have no space therebetween. Inone embodiment, the average roughness (Ra) of the composite structure102 is 0.2˜0.8 μm, 0.3˜0.8 μm, 0.40˜0.48 μm, 0.42˜0.45 μm or 0.425˜0.44μm, and the haze of the composite structure 102 is 15%˜29% or 15%˜25%,13%˜23% or 17%˜20%. The composite structure 102 in the present inventionhas the better combination/optimization of the range of the averageroughness (Ra) and the range of the haze to have better anti-Newton'sring performance (see following Table 1). Each second shape 102Y isconvex or concave relative to the corresponding first convex shape 102X(In the optical sheet 100A, each second shape 102Y is convex relative tothe corresponding first convex shape 102X; in the optical sheet 100B,each second shape 102Y is concave relative to the corresponding firstconvex shape 102X; and in the optical sheet 100C, a portion of thesecond shapes 102Y are convex relative to the corresponding first convexshape 102X and another portion of the second shapes 102Y are concaverelative to the corresponding first convex shape 102X.); however, thepresent invention is not limited to this case, for example, the secondshape(s) 102Y roughed or textured relative to the corresponding firstconvex shape 102X can be applied, such as dents, lines, cracks, pits andprotrusions, to increase diffusion effect. Each of the first convexshapes 102X has a larger smooth surface area (i.e. the density of thefirst convex shapes 102X is smaller) such that the slightsuperimposition/modification by the second shapes 102Y can destroy theregularity of the first convex shapes 102X but the composite structure102 corresponding to the combination of the first convex shapes 102X andthe second shapes 102Y can not obviously scatter light. Moreover,although the slight increment of the average roughness (Ra) resultingfrom the slight superimposition/modification by the second shapes 102Yincreases the haze of the composite structure 102, the haze of thecomposite structure 102 is low enough to eliminate the reflectiveNewton's ring and the loss of the brightness is slighter. The substrate103 has a light input surface 103A and a light output surface 103Bopposite to the light input surface 103A, wherein the compositestructure 102 is formed on the light input surface 103A. The compositestructure 102 can have a plurality of particles therein. In oneembodiment of the optical sheet 100A, 100C, each second shape 102Yconvex relative to the corresponding first convex shape 102X can beformed by the corresponding particles (e.g., the particles in theunitary or homogeneous film 101) protruding relative to thecorresponding first convex shape 102X.

The optical sheet 100A, 100B, 100C can further comprise a lightenhancement layer 105 disposed on the second surface 103B of thesubstrate 103. The light enhancement layer 105 can comprise a pluralityof microstructures 106 (e.g., prisms or microlens, preferably, eachmicrostructure is a prism). For example, the light enhancement layer 105comprises a plurality of prisms 106 extending substantially along afirst direction. The first direction can be the length direction of theprism 106, preferably perpendicular to the width direction (e.g., thecross-section direction) of the prism 106. The prism 106 can have alinear length, a meandering length or a wave length. The opposing facets106P, 106Q of each prism 106 intersect to form the ridge 106R (or peak,apex) of the prism 106. The facets of the adjacent prisms 106 intersectto form the groove 106G (or valley). The height of the ridge 106R of theprism 106 can varies along the first direction. Preferably, the dihedralangle defined by the opposing facets 106P, 106Q of the prism 106 can bethe same through the entire prism 106.

FIG. 3A to FIG. 3D illustrate a preferable manufacturing method forforming the optical sheet 100A, 100B, 100C in the present invention.However, the present invention is not limited to this case. Comparedwith single mold-cutting processes, the present invention adopts twodifferent mold-cutting processes to form the composite structure 102which has better anti-Newton's ring performance and better brightness.For convenience of explanation, only the manufacturing method forforming the optical sheet 100A is illustrated.

Mold-Cutting Processes I:

In the beginning, provide a mold 201 having a first surface 201A (seeFIG. 3A). Form a plurality of first concave shapes 202X on the firstsurface 201A of the mold 201 to change the first surface 201A of themold 201 to a second surface 201B of the mold 201 (see FIG. 3B).Preferably, the hard tool can be used to penetrate into the firstsurface 201A of the mold 201 to form a plurality of first concave shapes202X on the first surface 201A of the mold 201 (preferably, the mold 201is a roll used in the FTS (Fast Tool Server) process.). The hard toolcan be a knife having a smoothly-curved shape such that each of thefirst concave shapes 202X have a smoothly-curved shape (completelysmoothly-curved shape is preferable). Preferably, the front end of thehard tool has a round shape.

The first concave shapes 202X are configured for making correspondingconvex regions 203X having a first haze H₁ (see FIG. 4). The first hazeH₁ is 6%˜13%. In fact, the corresponding convex regions 203X may be notformed/embossed in the present invention and a structure merely forevaluating the first haze H₁. Each of the corresponding convex regions203X is lump-shaped for two dimensional light-diffusing. Each of thecorresponding lump-shaped convex regions 203X can have a smooth-curvedsurface (completely smoothly-curved shape is preferable); preferably,each of the corresponding convex regions 203X is a lenticular lens. Eachof the lenticular lenses 203X is ellipse-shaped and has a maximumdimension in the cutting direction of the hard tool. The adjacent convexregions or lenticular lenses 203X have a space therebetween; however,for increasing the diffusing area, it is preferable that the adjacentconvex regions or lenticular lenses 203X overlay so as to have no spacetherebetween. The average roughness (Ra) of the corresponding convexregions 203X is 0.2˜0.7 μm. In one embodiment, the average roughness(Ra) of the corresponding convex regions 203X is 0.25·0.6 μm. In thepreferred embodiment, the average roughness (Ra) of the correspondingconvex regions 203X is 0.3˜0.5 μm.

Mold-Cutting Processes II:

Form a plurality of second shapes 202Y on the plurality of first concaveshapes 202X to change the second surface 201B of the mold 201 to a thirdsurface 201C of the mold 201 (see FIG. 3C). Preferably, perform asandblasting process to form a plurality of second shapes 202Y on theplurality of first convex shapes 202X such that the second surface 201Bof the mold 201 is changed to the to the third surface 201C of the mold201; however, the present invention is not limited to this case, such asdeposition process. The average size of the first concave shapes 202X islarger than the average size of the second shapes 202Y. The density ofthe second shapes 202Y is larger than the density of the first concaveshapes 202X. Each of the second shapes 202Y is convex or concaverelative to the second surface 201B of the mold 201; however, thepresent invention is not limited to this case, for example, the secondshape(s) 202Y roughed or textured relative to the first concave shapes202X can be applied to the first concave shapes 202X, such as dents,lines, cracks, pits and protrusions, to increase diffusion effect.

Use the third surface 201C of the mold 201 to emboss a film 211 on asubstrate 213 to form a composite structure 212 corresponding to thecombination of the plurality of first concave shapes 202X and theplurality of second shapes 202Y (see FIG. 3D), wherein the compositestructure 212 has a second haze H₂. The substrate 213 has a light inputsurface 213A and a light output surface 213B opposite to the light inputsurface 213A, wherein the composite structure 212 is formed on the lightinput surface 213A. In the present invention, Mold-Cutting Processes I(the FTS process is preferred) is used to form the second surface 201Bof the mold 201 having a plurality of first concave shapes 202X(preferably, the first average roughness (Ra) R₁ of the correspondingconvex regions 203X is 0.2˜0.7 μm, 0.25˜0.6 μm or 0.3˜0.5 μm and thefirst haze H₁ is 6%˜13%) and Mold-Cutting Processes II (the sandblastingprocess is preferred) is used to slightly modify the first concaveshapes 202X. Each of the first concave shapes 202X of the second surface201B of the mold 201 has a larger smooth surface area (i.e. the densityof the first concave shapes 202X is smaller) such that the slightmodification by the second shapes 202Y can destroy the regularity of thefirst concave shapes 202X but the composite structure 212 correspondingto the combination of the first concave shapes 202X and the secondshapes 202Y can not obviously scatter light. Moreover, although theslight increment of the average roughness (Ra) resulting from the slightmodification by the second shapes 202Y increases the haze of thecomposite structure 212 (from the first haze H₁ to the second haze H₂),H₂/H₁ is low (e.g., less than 2.8) enough to eliminate the reflectiveNewton's ring and the loss of the brightness is slighter (e.g., the lossof the brightness is less than 5%, 4%, 3%, 2% or 1%) or H₂ is low(compared with the structure formed by single mold-cutting process)enough to eliminate the reflective Newton's ring and the loss of thebrightness is slighter (e.g., the loss of the brightness is less than5%, 4%, 3%, 2% or 1%). In one embodiment, H₂/H₁ is less than 2.5. In oneembodiment, H₂/H₁ is less than 2.25. In the preferred embodiment, H₂/H₁is less than 2. Also, although the slight modification by the secondshapes 202Y increases the average roughness (Ra) of the compositestructure 212 (from the first average roughness (Ra) R₁ to the secondaverage roughness (Ra) R₂), (R₂—R₁) is low (e.g., less than 0.2) enoughto eliminate the reflective Newton's ring and the loss of the brightnessis slighter (e.g., the loss of the brightness is less than 5%, 4%, 3%,2% or 1%). In one embodiment, (R₂−R₁) is less than 0.15. In oneembodiment, (R₂−R₁) is less than 0.1. In the preferred embodiment,(R₂−R₁) is less than 0.07. The second average roughness (Ra) R₂ of thecomposite structure 212 is 0.3˜0.8 μm or 0.40˜0.48 μm, preferably0.42˜0.45 μm, more preferably 0.425˜0.44 μm. The second haze H₂ is15%˜25%; preferably 13%˜23%, more preferably 17%˜20%. The compositestructure 212 formed by the process in the present invention hasdifferent characteristic about the combination/optimization of the rangeof the average roughness (Ra) and the range of the haze from thestructure formed by the single mold-cutting process to have betteranti-Newton's ring performance (see following Table 1). FIG. 5A to FIG.5B illustrate a real composite structure corresponding to thecombination of a plurality of first convex shapes and a plurality ofsecond shapes. FIG. 6 illustrates the composite structure can eliminateNewton's ring (especially reflective Newton's ring).

Experiments

The optical measurement result for the embossed film formed by differentprocesses are listed in Table 1. The embossed film of the Samples 1˜2are formed only by the FTS process. The embossed film of the Samples 3˜4are formed only by the sandblasting process. The embossed film 211/thecomposite structure 212 of the Samples 5-7 are formed by the process inthe present invention.

TABLE 1 Performance Performance of of Anti- Anti-Reflective-Transimissive- Bright- Ra Haze Newton's Newton's ness (μm) (%) Ring Ring(%) Sample 1 0.390 7.5~10  General Worse 100  Sample 2 0.610 35 WorseBetter 95 Sample 3 0.120 10 Worse Worse 98 Sample 4 0.310 34 BetterBetter 93 Sample 5 0.420 13~15 General General 98~101 Sample 6 0.430 18Better Better 97~100 Sample 7 0.450 23 Better Better 95~98 

From Table 1; for the embossed film of the Samples 3˜4 formed only bythe sandblasting process, increasing the average roughness (Ra) caneliminate the transimissive Newton's ring and the reflective Newton'sring, but the brightness drops a lot from 98% to 93%; for the embossedfilm of the Samples 1˜2 formed only by the FTS process, increasing theaverage roughness (Ra) or the haze can eliminate the transimissiveNewton's ring and slightly reduce the brightness, but the reflectiveNewton's ring occurs. By the process in the present invention, theembossed film 211/the composite structure 212 of the Samples 5-7 hasboth the better performance of anti-Newton's ring and brightness.Specifically speaking, after performing Mold-Cutting Processes I (theFTS process is preferred), perform Mold-Cutting Processes II (thesandblasting process is preferred) is used to slightly modify the firstconcave shapes 202X such that the average roughness (Ra) of the embossedfilm 211/the composite structure 212 corresponding to the combination ofthe first concave shapes 202X and the second shapes 202Y is modified to0.40˜0.48 μm, preferably 0.42˜0.45 μm (the corresponding haze is13%˜23%), more preferably 0.425˜0.44 μm, to have both the betterperformance of anti-Newton's ring and brightness.

The above disclosure is related to the detailed technical contents andinventive features thereof. People skilled in this field may proceedwith a variety of modifications and replacements based on thedisclosures and suggestions of the invention as described withoutdeparting from the characteristics thereof. Nevertheless, although suchmodifications and replacements are not fully disclosed in the abovedescriptions, they have substantially been covered in the followingclaims as appended.

What is claimed is:
 1. An optical sheet, comprising: a substrate havinga first surface and a second surface opposite to the first surface; anda film having a third surface and a fourth surface opposite to the thirdsurface, wherein the third surface of the film is on the first surfaceof the substrate and the fourth surface of the film comprises astructure corresponding to a combination of a plurality of first convexshapes and a plurality of second convex or concave shapes superimposedon the plurality of first convex shapes, wherein the average size of theplurality of first convex shapes is larger than the average size of theplurality of second convex or concave shapes, wherein the averageroughness (Ra) of the structure is 0.42˜0.45 μm.
 2. The optical sheetaccording to claim 1, wherein the first surface of the substrate is alight input major surface and the second surface of the substrate is alight output major surface.
 3. The optical sheet according to claim 1,wherein the average roughness (Ra) of the structure is 0.425˜0.44 μm. 4.The optical sheet according to claim 1, wherein the haze of thestructure is 13%˜23%.
 5. An optical sheet, comprising: a substratehaving a first surface and a second surface opposite to the firstsurface; and a film having a third surface and a fourth surface oppositeto the third surface, wherein the third surface of the film is on thefirst surface of the substrate and the fourth surface of the filmcomprises a structure corresponding to a combination of a plurality offirst convex shapes and a plurality of second convex shapes superimposedon the plurality of first convex shapes, wherein the average size of theplurality of first convex shapes is larger than the average size of theplurality of second convex shapes, wherein the average roughness (Ra) ofthe structure is 0.42˜0.45 μm.
 6. The optical sheet according to claim5, wherein the first surface of the substrate is a light input majorsurface and the second surface of the substrate is a light output majorsurface.
 7. The optical sheet according to claim 5, wherein the averageroughness (Ra) of the structure is 0.425˜0.44 μm.
 8. The optical sheetaccording to claim 5, wherein the haze of the structure is 13%˜23%. 9.An optical sheet, comprising: a substrate having a first surface and asecond surface opposite to the first surface, wherein the first surfaceof the substrate is a light input major surface and the second surfaceof the substrate is a light output major surface; a light enhancementlayer disposed on the second surface of the substrate; and a film havinga third surface and a fourth surface opposite to the third surface,wherein the third surface of the film is on the first surface of thesubstrate and the fourth surface of the film comprises a structurecorresponding to a combination of a plurality of first convex shapes anda plurality of second convex or concave shapes superimposed on theplurality of first convex shapes, wherein the average size of theplurality of first convex shapes is larger than the average size of theplurality of second convex or concave shapes, wherein the averageroughness (Ra) of the structure is 0.42˜0.45 μm.
 10. The optical sheetaccording to claim 9, wherein the light enhancement layer comprise aplurality of microstructures each of which extends substantially along afirst direction.
 11. The optical sheet according to claim 9, wherein thelight enhancement layer comprise a plurality of prisms each of whichextends substantially along a first direction.
 12. The optical sheetaccording to claim 9, wherein the density of the second convex orconcave shapes is larger than the density of the first convex shapes.13. The optical sheet according to claim 9, wherein the averageroughness (Ra) of the structure is 0.425˜0.44 μm.
 14. The optical sheetaccording to claim 9, wherein the haze of the structure is 13%˜23%. 15.An optical sheet, comprising: a substrate having a first surface and asecond surface opposite to the first surface; a light enhancement layerdisposed on the second surface of the substrate; and a film having athird surface and a fourth surface opposite to the third surface,wherein the third surface of the film is on the first surface of thesubstrate and the fourth surface of the film comprises a structurecorresponding to a combination of a plurality of first convex shapes anda plurality of second convex or concave shapes superimposed on theplurality of first convex shapes, wherein the plurality of first convexshapes of the structure are formed by film-embossing of a mold and eachof the plurality of first convex shapes of the structure has acorresponding recess on the mold, wherein the plurality of recesses onthe mold are cut by a knife having a smoothly-curved shape in a FTS(Fast Tool Server) process, wherein the average size of the plurality offirst convex shapes is larger than the average size of the plurality ofsecond convex or concave shapes, wherein the average roughness (Ra) ofthe structure is 0.42˜0.45 μm.